Liquid crystal compounds, mixtures and devices

ABSTRACT

Compounds for use in ferroelectric smectic liquid crystal mixtures of general formula (I), where R 1  is alkyl, Y is COO or OOC, m, n and p are 0 or 1 provided m+n is 2 or less and when Y is OOC, p is 0, X is a mesogenic group, especially 4 1  -alkyl- or alkyl-biphenyl or bicyclohexyl. Methods for preparation of these compounds are given, and ferroelectric smectic liquid crystal mixtures containing them are described.

The invention relates to novel compounds which may be used asconstituents of ferroelectric smectic liquid crystal mixtures, to suchmixtures containing these compounds, and to electro-optical deviceswhich use these mixtures.

Ferroelectric smectic liquid crystal materials use the ferroelectricproperties of the chiral tilted smectic C, F, G, H, I and K phases(designated S_(C) * etc, the asterisk denoting chirality). The S_(C)phase is generally the most useful, as it is the most fluid, and it isparticularly desirable that the material shows an S_(A) or nematic(denoted N) phase at temperatures above the chiral smectic phase, toassist surface alignment in a liquid crystal device. Ferroelectricsmectic liquid crystal materials desirably have low viscosity, S_(C)phases that persist over a broad temperature range which includesambient temperature and have a long helical pitch, chemical andphotochemical stability, and a high spontaneous polarisationcoefficient, Ps, measured in nC cm⁻². Such materials offer thepossibility of very fast switched liquid crystal display devices, withbistable characteristics.

Although some single compounds show all of the desirable propertiesoutlined above, ferroelectric smectic liquid crystal materials in usetoday generally consist of a mixture of compounds at least one of whichis an optically active compound, i.e. containing an asymmetricallysubstituted carbon atom. This asymmetric centre is generally combinedwith a mesogenic molecular `core`. A principal function of the opticallyactive compound in the mixture is to induce the mixture to show ahelical smectic phase, e.g. S_(C) *, and to have a high Ps.

A wide variety of such compounds is known, but it is not usual to findcompounds which result in S_(c) * mixtures with all of the abovementioned desirable properties. For example the alpha-hydroxycarboxylicacid derivatives described in PCT/GB85/00512 which contain theasymmetric centre: ##STR1## form S_(c) * mixtures with a very high Psbut a very short helical pitch.

It is an object of the invention to explore the possibilities of otherasymmetric centres, to provide optically active compounds of increasedusefulness in ferroelectric smectic liquid crystal mixtures.

According to the invention a novel compound has a formula I: ##STR2##where R₁ is alkyl, Y is selected from --COO-- or --OOC--; m, n and p areindependently 0 or 1, provided that (m+n+p) is no more than 2, and whenY is OOC then p is 0; and X is a mesogenic group.

The term "mesogenic" used herein with reference to the group X indicatesthat X is one of the molecular "cores" composed of chains of linkedcyclic group that induce the molecule to show liquid crystal phases orto be miscible with liquid crystal materials, particularly those showinga smectic C liquid crystal phase, i.e. `smectogenic`.

Many such mesogenic or smectogenic cores are known, for example thegroups shown attached to the group --COOR* in EP-A-0110299, and thegroups represented by the symbol Q in U.S. Pat. No. 4,613,209.

The asterisk in formula I indicates the asymmetric centre.

Also the invention provides a ferrolectric smectic liquid crystalmaterial, being a mixture of compounds, at least one of which is acompound of formula I in an optically active or racemic form, preferablythe former. The structural preferences for the compound of formula Idiscussed below are based largely on suitability and desirability foruse in such mixtures.

Preferably R₁, in formula I is C₁ -C₅ n-alkyl, especially methyl.

Preferred structures for compounds of formula I are therefore thoselisted in table 1 below.

                  TABLE 1                                                         ______________________________________                                         ##STR3##                 1.1                                                  ##STR4##                 1.2                                                  ##STR5##                 1.3                                                  ##STR6##                 1.4                                                  ##STR7##                 1.5                                                  ##STR8##                 1.6                                                  ##STR9##                 1.7                                                  ##STR10##                1.8                                                  ##STR11##                1.9                                                 ______________________________________                                    

Particularly preferred structures are 1.1, 1.2, 1.3, 1.5 and 1.7 listedin table 1.

The advantages of the invention derive primarily from the nature of thenovel asymmetric group in formula I and the nature of the mesogenicgroup X is less critical, although for use in ferroelectric smecticliquid crystal mixtures the group X is preferably a smectogenic group.The group X may therefore be a known smectogenic group.

Some preferred groups X are listed in table 2, R is alkyl or alkoxycontaining 1 to 12 carbon atoms. The phenyl rings shown in table 2 maybe substituted, as shown with respect to groups 2.1, 2.2 and 2.3 intable 2 for example. In table 2 ##STR12## represents trans cyclohexyland (F) indicates that the group may carry one or more fluorinesubstituents on one or more of the rings shown.

In table 2, R is preferably n-alkyl or n-alkoxy containing 3 to 10carbon atoms.

Structures 2.1, 2.2 and 2.4 are particularly preferred.

Compounds of formula I may be made by a variety of synthetic routes, forexample the routes A and B shown schematically in FIGS. 1 and 2. Route Astarts from commercially available 2-hydroxy-2-phenyl propionic acid. Itwill be apparent that the individual steps use well known-chemicalreaction although the products and overall routes are novel.

                  TABLE 2                                                         ______________________________________                                         ##STR13##               2.1                                                   ##STR14##               2.2                                                   ##STR15##               2.3                                                   ##STR16##               2.4                                                   ##STR17##               2.5                                                   ##STR18##               2.6                                                   ##STR19##               2.7                                                   ##STR20##               2.8                                                   ##STR21##               2.9                                                   ##STR22##               2.10                                                  ##STR23##               2.11                                                  ##STR24##               2.12                                                  ##STR25##               2.13                                                  ##STR26##               2.14                                                  ##STR27##               2.15                                                  ##STR28##               2.16                                                 ______________________________________                                    

A ferroelectric smectic liquid crystal mixture of the invention containsat least one compound of formula I. Preferably the compound of formula Iis in an optically active form. Typically the mixture contains 1-20 wt %of the compound of formula I, for example around 10% or less. Theinclusion of a compound of formula I in the mixture offers theadvantages of a high Ps combined with a very long helical pitch of theS_(C) * phase. This long pitch is valuable because in many common typesof ferroelectric smectic display device it is desirable to have ahelical pitch length comparable with the spacing of the electrodes ofthe device, which is generally around 1-5μ. An example of such a deviceis the "Clark Lagerwall device" described in App. Phys. Lett. 36,(1980), 899.

A ferroelectric smectic liquid crystal mixture according to theinvention contains at least one compound of formula I. Typically themixture will contain 1-20% by weight of the compound of formula I, e.g.around 10% or less, in an optically active enantiomeric form.

The mixture also contains one or more compounds which either separatelyor together show an S_(C) phase. Such compounds are known as smectichosts.

A large number of classes of compounds which may be used as smectichosts are known, and some examples of suitable classes are listed intable 3. Of these compounds those of formula 3.1, 3.2, 3.3 and 3.4 arepreferred, especially where R^(A) and R^(B) independently contain 5-10carbon atoms. It is particularly preferred to use a mixture of two ormore members, e.g. homologues, of the same class for example to reducethe melting point.

These preferred hosts allow the possibility of Sc mixtures showing an Scphase persisting over a wide temperature range including roomtemperature, and also an S_(A) phase at a temperature above the Sc, toassist in alignment of the liquid crystal material. Typically themixture will contain 40-99% of host compounds, e.g. around 90-95 wt %.

                                      TABLE 3                                     __________________________________________________________________________    3.1                                                                               ##STR29##                          3.7                                                                               ##STR30##                          3.2                                                                               ##STR31##                          3.8                                                                               ##STR32##                          3.3                                                                               ##STR33##                                                                 3.4                                                                               ##STR34##                                                                 3.5                                                                               ##STR35##                                                                 3.6                                                                               ##STR36##                                                                 __________________________________________________________________________

where R^(A) and R^(B) are C₁₋₁₂ n-alkyl or n-alkoxy.

Additives in such a mixture may serve a number of functions. One suchfunction is as "pitch compensators." "Pitch compensation" is theinclusion in the ferroelectric smectic mixture of two or more compoundswhich induce the appearance of helical smectic phases of opposite twistsense. In such a case the compounds will unwind the helical phaseinduced by the other. This may be used to produce a long pitch helicalsmectic phase, and by the controlled use of appropriate quantities ofthe two compounds the pitch of the mixture may be closely controlled.

In mixtures according to the invention, pitch compensation may beachieved conveniently by using opposite-twisting compounds of formula Ias discussed above which may be different compounds of formula I orenantiomers e.g. the L(+) and S(+) forms of such a compound.

Alternatively or additionally other optically active additives may beused, for example the amides described in PCT/GB 87/00223, e.g. any ofthose listed in table 6 thereof, or the terphenyls described in UKPatent Application 8703103, the latter being particularly suitable whenthe host includes one or more compounds of formula 3.3.

Examples of pitch compensators of these two types include the compounds:##STR37## where R and R' are independently C₁₋₁₂ n-alkyl or n-alkoxy. R'is preferably CH₃. Many other pitch compensating additives will beapparent to those skilled in the art. For example when the host containsone or more compounds of formula 3.1 or 3.2 a suitable pitch compensatormay be a compound of formula 3.1 or 3.2 where R^(B) or/and R^(A) areoptically active alkyl or alkoxy. Compounds of formula I frequently formhelical S_(C) phases of such long pitch in a smectic mixture that pitchcompensation is unnecessary.

Additives may be used to encourage the appearance of an S_(A) phase at atemperature above that at which the S_(C) * phase persists, to assist inalignment of the S_(C) * phase during assembly of the service. When thehost consists largely of compounds of formula 3.1 or 3.2 preferredadditives to achieve this have a formula: ##STR38## where R and R' areindependently n-alkyl or n-alkoxy containing 1-12 carbon atoms,especially 5-9.

Additives may alternatively serve the function of suppressingundesirable smectic phases such as S_(A) or S_(B) so that these appear,if at all, as far as possible from the working temperature range. Apreferred class of additives fulfilling this function are the esters offormula: ##STR39## where R and R' are independently n-alkyl or n-alkoxycontaining 1-12 carbon atoms, especially 5-9. Compounds of formula 6 mayalso be used as additives to suppress undesirable smectic phases inmixtures where they are not used as hosts.

Additives may also improve other properties of the mixture, for exampleto increase the switching speed. Compounds which do this include thosewhich contain a ##STR40## group, for example those of general formula 6##STR41## where R and R' are independent alkyl groups especiallycontaining 3 to 10 carbon atoms.

Other known additives may be included, for example to improve viscosity,melting point or other properties.

Typically a mixture according to the invention may contain 0-50 wt % ofadditives, preferably 0-20%. Only minute amount of pitch compensatorsmay be necessary, e.g. less than 1 wt % of the tight twisting compound4.1 where R is C₁₀ H₂₁ and R¹ is CH₃, and preferably no more than about10 wt % of compounds such as 5, 6 7.1 or 7.2.

A mixture according to the invention may be used in any of the knowntypes of ferroelectric smectic liquid crystal electrooptic device, and adevice incorporating such a mixture is another aspect of the invention.

The invention will now be described by way of example only withreference to the accompanying drawings, which show:

FIGS. 1 and 2: preparative routes for compounds of the invention.

FIG. 3: a liquid crystal device.

EXAMPLE 1 A. Preparation via route A of compounds of Formula ##STR42##

Step 1

An intimate mixture of L-(+)-mandelic acid (1.52 g 10 m mol) and silver(I) oxide (4.6 g, 20 m mol) was added to cooled (ice-salt) methyl iodide(5.68 g, 40 m mol) batchwise during 1 hour, excluding light throughout.The mixture was heated under reflux for 2 hours, then it was cooled,diluted with ether (50 ml) and the inorganic salts removed byfiltration. Removal of the solvent gave a pale yellow oil.

Step 2

The oil from step 1 was suspended in a mixture of aqueous 10% potassiumhydroxide (100 ml) and methanol (30 ml), and heated to 70° C. for 4hours. The solution was then cooled, acidified, and extracted with ethylacetate (3×100 ml). The combined organic extracts were shaken withaqueous 10% sodium thiosulphate (100 ml) and dried over MgSO₄. Removalof the solvent gave a white solid which was purified by recrystalisationfrom petroleum fraction (bp 60°-80° C.) Yield=1.41 g, 85%. [α]_(D) ²⁰ inalcohol was +150°. Melting point was 63°-64° C.

Step 3

The O-methyl L-(+)-mandelic acid from step 2 above (1-66 g, 10 m mol),the appropriate alcohol or phenol (11 m mol), DCC (2.27 g, 11 m mol) and4-pyrrolidinopyridine (0.148 g, 1 m mol) were stirred in sieve-drieddichloromethane (50 ml) at room temperature until the reaction seemed tobe complete by tlc (silica gel, dichloromethane:petroleum ether 2:1).The precipitated dicyclohexylurea was filtered off and the filtrate waswashed successively with water (3×50 ml) aqueous 5% acetic acid (3×50ml), water (3×50 ml) then dried over MgSO₄. Removal of the solvent gavea brown residue which was purified by column chromatography on silicagel, using dichloromethane:petroleum fraction (bpt 60°-80° C.) aseluant, followed by recrystallisation from petroleum fraction (bpt60°-80° C.).

Using this method the following compounds were prepared:

    __________________________________________________________________________    Compound                     Yield                                                                             m pt                                                                             [α].sub.D.sup.23                    __________________________________________________________________________     ##STR43##                   72%  69°                                                                      +84.6°                              ##STR44##                   76%  44°                                                                      +82.7°                              ##STR45##                   65%  58°                                                                      +83.4°                              ##STR46##                       <20°                                                                      +7.15°                              ##STR47##                       <20°                                                                      +21.0°                             __________________________________________________________________________

B. Liquid Crystal Properties

(i) Compound ##STR48##

A 25 mole % solution of this compound in the smectic mixture H1, havingthe composition: ##STR49## experienced a S_(C) -S_(A) transition at58.1° C. The Ps value of this compound, extrapolated to 100%, wasmeasured at various temperatures in H1.

The results are tabulated below, where T refers to the temperature belowthe S_(C) -S_(A) transition.

    ______________________________________                                               T      Ps                                                              ______________________________________                                               -10° C.                                                                       92.5                                                                   -20° C.                                                                       136.2                                                                  -30° C.                                                                       170.8                                                           ______________________________________                                    

(ii) Measurement of helical pitch.

Indications of the possibility of inducement of a long pitch helicalsmectic phase by the compounds were made by measurement of the pitchlength of cholesteric phases produced by mixing the compounds with thenematic material E7 (BDH Ltd., Broom Road, Poole, Dorset, UK) which hasthe composition:

    ______________________________________                                         ##STR50##                 51    wt %                                          ##STR51##                 25    wt %                                          ##STR52##                 16    wt %                                          ##STR53##                 8     wt %                                         ______________________________________                                    

(It should be noted that the formation of a long pitch cholesteric phaseby a compound in a nematic mixture is an accepted indication of theability of the compound to form long pitch helical phases).

The results were as follows:

    ______________________________________                                                              extrapolated pitch                                      ______________________________________                                         ##STR54##              0.63μ                                               ##STR55##              0.95μ                                              ______________________________________                                    

It is useful to compare these pitch lengths with hose produced in E7 bythe smectic lactate compounds described in PCT/GB85/00512:

    __________________________________________________________________________                                          extrapolated pitch                      __________________________________________________________________________     ##STR56##                            0.076μ                                ##STR57##                            0.096μ                                ##STR58##                             0.14μ                               __________________________________________________________________________

The very considerably longer pitch induced by the mandelic acidderivatives of the present invention is thus immediately apparent.

EXAMPLE 2 Preparation via route A of compounds of Formula ##STR59## Step4

Borane-tetrahydrofuran complex, (13 cm³ of a 1.0M solution intetrahydrofuran, 13 mmol), was added slowly to a stirred solution ofn-nonyl or n-octyloxy biphenyl carboxylic acid (10 mmol) in drytetrahydrofuran (100 cm³) at 0° C. under an atmosphere of dry nitrogen.

The solution was stirred for 3 h and the reaction was quenched by theaddition of a 1:1 mixture of water and tetrahydrofuran (90 cm³). Theresulting solution was saturated with potassium carbonate, the layerswere separated and the aqueous layer was shaken with ether (2×50 cm³).The combined extracts were dried and evaporated to give the crudeproduct, which was recrystallised from industrial methylated spirit togive the pure product as white needles. ##STR60##

Step 5

The carboxylic acid from Step 2 (10 mmol), the appropriate alcohol fromStep A3 (11 mmol), dicyclohexyl-carbodiimide (DCC) (11 mmol) and4-pyrrolidinopyridine (1 mmol) were stirred in sieve-drieddichloromethane (50 ml) at room temperature until the reaction seemed tobe complete by tlc. The precipitated dicyclohexylurea was filtered offand the filtrate was washed successively with water (3×50 ml) aqueous 5%acetic acid (3×50 ml), water (3×50 ml) then dried over MgSO₄. Removal ofthe solvent gave a brown residue which was purified by columnchromatography on silica gel, using dichloromethane:petroleum fraction(bpt 60°-80° C.) as eluant, followed by recrystallisation from petroleumfraction (bpt 60°-80° C.).

The following compounds were prepared in this way: ##STR61##

EXAMPLE 3

Preparation of compounds of structure: ##STR62## by route B.

Step 12

A solution of (S)-(-)-2-hydroxy-3-phenylpropionic acid (10 mmol), in amixture of methanol (50 cm³) and conc.sulphuric acid (0.5 cm³), washeated under reflux for 2 h. The solution was cooled and the solvent wasremoved under reduced pressure. The residue was dissolved in ethylacetate (50 cm³) and this solution was shaken successively withsaturated aqueous sodium chloride (50 cm³), saturated aqueous sodiumhydrogen carbonate (50 cm³) and saturated aqueous sodium chloride (50cm³). The organic phase was dried and evaporated to give the curdeproduct which was recrystallised from petroleum fraction (b.p. 40°-60°C.) to give the pure product (98%) as white needles.

m.p. 48°-49° C., [α]=-7.2° in chloroform.

The product was identified as: ##STR63##

Step 13

A solution of methyl (S)-(-)-2-hydroxy-3-phenylpropionate from step 12(10 mmol) in dry tetrahydrofuran (50 cm³) was added, during 1 hr, to acooled, stirred suspension of sodium hydride (11 mmol) in drytetrahydrofuran (75 cm³) under an atmosphere of dry nitrogen. Stirringwas continued for 30 min. Dry methyl iodide (11 mmol) was added during15 min. and the mixture was heated to 40° C. for 1 hr.

The resulting suspension was cooled, excess sodium hydride was destroyedby the addition of "wet" tetrahydrofuran (50 cm³) and the mixture waspoured into ice (100 g). The solution was saturated with potassiumcarbonate and the organic layer was separated. The aqueous layer wasshaken with ether (2×50 cm³) and the combined extracts were dried.Evaporation of the solvent followed by distillation under reducedpressue gave the pure product (90%) as a colourless oil b.p. 172°-175°C. C @ 0.2 mmHg. [α]=-12.4°.

The product was identified as: ##STR64##

Step 14

A mixture of methyl (S)-(-)-2-methoxy-3-phenylpropionate from step 13(10 mmol), aqueous 10% potassium hydroxide (50 cm³) and methanol (15cm³) was stirred overnight at room temperature. The solution was cooled,acidified by the addition of dil. hydrochloic acid and saturated withsodium chloride. The resulting mixture was shaken with ethyl acetate(2×50 cm³), the combined extracts were dried and the solvent wasevaporated to give the crude product. This was purified by columnchromatography on silica gel in a 5:1 mixture of dichloromethane andmethanol to give the pure product as a colourless oil (95%) [α]=-17.7°in chloroform. The dicyclohexylamine salt had m.p. 122°-124° C.

The product was identified as: ##STR65##

Step 15

This was carried out using a scheme analogous to that of step 3 with theacid of step 14 and the appropriate alcohol or phenol. The followingproducts were prepared: ##STR66##

EXAMPLE 4

Preparation of compounds of structure: ##STR67## by route A.

Step 6

Borane-tetrahydrofuran complex (13 ml of a 1.0M solution intetrahydrofuran; 13 mmol) was added slowly to a stirred solution of(S)-(+)-2-methoxy-2-phenylacetic acid (10 mmol) prepared via step 2 indry tetrahydrofuran at 0° C. under an atmosphere of dry nitrogen.

The solution was stirred for 3 h and the reaction was quenched by theaddition of a 1:1 mixture of water and tetrahydrofuran (90 ml). Theresulting solution was aturated with potassium carbonate, the layerswere seprated and the aqueous layer was shaken with ether (2×50 ml). Thecombined extracts were dried and evaporated to give the crude product(84%) as a colourless oil, b.p. 130°-133° @ 0.005 mmHg. [α]=+118° inchloroform.

The product was identifed as: ##STR68##

Step 7

Esterification of the product of step 6 with n-octyloxy biphenylcarboxylic acid was carried out using a scheme analogous to that of step3 above. ##STR69## was prepared mp=68° C. [α]_(D) ²⁶ =+6.5°.

EXAMPLE 5

Further examples of ferroelectric smectic liquid crystal mixtures of theinvention are listed below.

Mixture 5A ##STR70## was made into a mixture with the smectic mixture H1(identified in 1.B.(i) above). The Ps was measured and extrapolated to100% at two temperatures T below the S_(C) -S_(A) transition.

    ______________________________________                                               T    Ps                                                                ______________________________________                                               -10°                                                                        14.9 nCcm.sup.-2                                                         -20°                                                                        27.0 nCcm.sup.-2                                                  ______________________________________                                    

Mixture 5B ##STR71## Ps was measured at two temperatures T below theS_(C) -S_(A) transition in a mixture consisting of a mixture of:##STR72## and extrapolated to 100% of the compound.

    ______________________________________                                               T    Ps                                                                ______________________________________                                               -10°                                                                         67 nCcm.sup.-2                                                          -20°                                                                        100 nCcm.sup.-2                                                   ______________________________________                                    

Mixture 5C

The compound of the invention used in mixture 5B was made into a 10 wt %solution in a compound of formula 3.5 of table 3 with R^(A) being C₈H₁₇. Liquid crystal transitions (°C.) were:

    S.sub.? →50→S.sub.C →74.3→S.sub.A →101.3→I(116.7N)

Ps extrapolated to 100% at the temperatures (°C.) indicated, and tiltangle (°) of the S_(C) phase of the mixture were:

    ______________________________________                                        T               Ps     Tilt                                                   ______________________________________                                        60              144    16                                                     64.3            96     12                                                     70              72      4                                                     ______________________________________                                    

Mixture 5D

The compound of the invention used in mixture 5B was made into a 5 wt %solution in H1 (see example 1B(i)). Liquid crystal transitions were:

    S.sub.? →23.8→S.sub.C →94.2→S.sub.A →98.8→N*→142→I

Ps extraplated to 100% at the temperatures indicated was:

    ______________________________________                                        T               Ps     Tilt                                                   ______________________________________                                        30              164    11.5                                                   50              130    11.5                                                   70               80    11.5                                                   84.2             44    10.0                                                   ______________________________________                                    

Mixture 5E ##STR73## was made into a 3 wt % solution in H1. Liquidcryatal transitions were:

    25→S.sub.C →95→S.sub.A →102→N→146.5→I

Ps extrapolated to 100% at the temperatures indicated was:

    ______________________________________                                        T               Ps    Tilt                                                    ______________________________________                                        50              83    9.5                                                     70              53    9.0                                                     85              28    7.0                                                     ______________________________________                                    

Mixture 5F

The propylbicyclohexyl compound prepared in example 1 was made up into a5 wt % solution in H1. Liquid crystal transitions were:

    Solid→28→S.sub.C →77→S.sub.A →96→N→142→I

Ps extrapolated to 100% and tilt at the temperatures indicated were:

    ______________________________________                                        T               Ps    Tilt                                                    ______________________________________                                        30              60    15                                                      50              40    12                                                      60              28    9.5                                                     77              24    7.0                                                     ______________________________________                                    

An example of the use of a compound of Formula I in a liquid crystalmaterial and device embodying the present invention will now bedescribed with reference to FIG. 3.

In FIG. 3 a liquid crystal cell comprises a layer 1 of liquid crystalmaterial exhibiting a chiral smectic phase sandwiched between a glassslide 2 having a transparent conducting layer 3 on its surface, e.g. oftin oxide or indium oxide, and a glass slide 4 having a transparentconducting layer 5 on its surface. The slides 2, 4 bearing the layers 3,5 are respectively coated by films 6, 7 of a polyimide polymer. Prior toconstruction of the cell the films 6 and 7 are rubbed with a soft tissuein a given direction the rubbing directions being arranged parallel uponconstruction of the cell. A spacer 8 e.g. of polymethyl methacrylate,separates the slides 2, 4 to the required distance, e.g. 5 microns. Theliquid crystal material 1 is introduced between the slides 2, 4 to therequired distance, e.g. 2 microns. The liquid crystal material 1 isintroduced between the slides 2, 4 by filling the space between theslides 2, 4 and spacer 8 and sealing the spacer 8 in a vacuum in a knownway. Preferably the liquid crystal material is in the smectic A, nematicor isotropic liquid phase (obtained by heating the material) when it isintroduced between the slides 2, 4 to facilitate alignment of the liquidcrystal molecules with the rubbing directions on the slides 2, 4.

A polarizer 9 is arranged with its polarization axis parallel to therubbing direction on the films 6, 7 and an analyzer (crossed polarizer)10 is arranged with its poarization axis perpendicular to that rubbingdirection.

When a square wave voltage (from a conventional source not shown)varying between about +10 volts and -10 volts is applied across the cellby making contact with the layers 3 and 5 the cell is rapidly switchedupon the change in sign of the voltage between a dark state and a lightstate as explained above.

In an alternative device (not shown) based on the cell constructionshown in FIG. 3 the layers 3 and 5 may be selectively shaped in a knownway, e.g. by photoetching or deposition through a mask, e.g. to provideone or more display symbols, e.g. letters, numerals, words or graphicsand the like as conventionally seen on displays. The electrode portionsformed thereby may be addressed in a variety of ways which includemultiplexed operation.

I claim:
 1. A compound having a formula: ##STR74## where R₁ is methyl, Y is COO or OOC, m, n and p are independently 0 or 1, provided that (m+n+p) is no more than 2, and when Y is OOC then p is 0, and wherein x is selected from: ##STR75## where R is C₁₋₁₂ alkyl or alkoxy and (F) indicates that the phenyl ring indicated may carry a lateral fluorine substituent.
 2. A compound according to claim 1 having a formula: ##STR76## where p is 0 or
 1. 3. A compound according to claim 2 having a formula: ##STR77## where R is C₃ to C₁₀ n-alkyl or n-alkoxy.
 4. A compound according to claim 2 having a formula: ##STR78## where R is C₃ to C₁₀ n-alkyl or n-alkoxy.
 5. A compound according to claim 2 having a formula: ##STR79## where R is C₃ to C₁₀ n-alkyl or n-alkoxy.
 6. A compound according to claim 2 having a formula: ##STR80## where R is C₃ to C₁₀ n-alkyl or n-alkoxy.
 7. A compound according to claim 1 having a formula: ##STR81##
 8. A compound according to claim 7 having a formula: ##STR82## where R is C₃ to C₁₀ n-alkyl or n-alkoxy.
 9. A compound according to claim 1 having a formula: ##STR83##
 10. A compound according to claim 9 having a formula: ##STR84## where R is C₃ to C₁₀ n-alkyl or n-alkoxy.
 11. A compound according to claim 1 having a formula: ##STR85## where R is C₃ to C₁₀ n-alkyl or n-alkoxy.
 12. A compound according to claim 11 having a formula: ##STR86## where R is C₃ to C₁₀ n-alkyl or n-alkoxy.
 13. A ferroelectric smectic liquid crystal material being a mixture of compounds, characterised in that at least one of the said compounds is a compound of formula I below: ##STR87## where R₁ is alkyl, Y is selected from --COO-- or --OOC--; m, n and p are independently 0 or 1, provided that (m+n+p) is no more than 2, and when Y is OOC then p is 0; and x is selected from ##STR88## the compound of formula I being in an optically active or racemic form.
 14. A ferroelectric smectic liquid crystal material being a mixture of compounds, characterised in that at least one of the said compounds is a compound as claimed in claim 1 in an optically active or racemic form.
 15. A material according to claim 14, characterised that in addition to the compound as claimed in any one of claims 1 to 12 it also contains at least one compound of formula: ##STR89## where R^(A) and R^(B) are independently C₁ to C₁₂ n-alkyl or n-alkoxy.
 16. An electro-optical display device characterised in that it uses a liquid crystal material as claimed in claim
 13. 